refactor(client): split reporting/scan/manifest out of client_send

Move the stats/progress reporting, scanner-preparation/scan helpers and
manifest/list/dry-run senders out of the ~3.9k-line client_send.c into
client_report.c, client_scan.c and client_manifest.c, sharing declarations
through the new internal client_send_internal.h.  client_send.c keeps the
transfer orchestration and is now ~2.1k lines.

Decompose the monolithic send_files into static phase helpers
(send_files_prepare/_prepare_delete/_run/_finalize/_cleanup) driven by a
single SendFilesState; ownership, ordering and exit codes are unchanged.

No behavior change.

CMakeLists.txt

@@ -136,6 +136,9 @@ set(SERVER_MAIN_SRCS src/server/server.c)
 # Client implementation (no main): everything except the CLI entry point.
 set(CLIENT_CORE_SRCS
 	src/client/change_list.c
+	src/client/client_manifest.c
+	src/client/client_report.c
+	src/client/client_scan.c
 	src/client/client_send.c
 	src/client/client_validation.c
 	src/client/scanner.c